This invention relates to a process for the recovery of oil from oil-bearing shale and for employing the oil-depleted material as a combustible heat source in the process. As is pointed out in U.S. Pat. No. 3,325,395, extremely large deposits of oil-bearing shale are known to exist in the United States, and particularly in Colorado and Wyoming. Shale rock and certain oil sands contain a substance called "kerogen" or "petrogen", which is an organic, waxy compound. When heated to a temperature of about 800.degree. F., and in a nonoxidizing atmosphere, it cracks partially to yield a substance which has the properties resembling crude oil. In fact, any organic or organic-containing material will yield such a substance when treated in this manner.
The aforementioned patent presents two basic species of oil shale treatment. One aspect of the heated draft recycle system in that patent involves the partial combustion by an air fuel of the circulating retort draft stream which comes from the cooling zone and is used for sustaining the heating cycle in the retorting zone. Another aspect of the heated draft recycle system involves the heated recycle draft-air draft combustion system which embodies partial in situ combustion of the shale undergoing retorting. The first-mentioned system, while having many preferred aspects, results in the dilution of the shale gas by the air fuel, so that the shale gas is not usable as a fuel byproduct of the process. It has been found that the B.T.U. content of the shale gas is reduced from about 400 B.T.U. to about 100 B.T.U., due to dilution by the air fuel. The second-mentioned system also has disadvantages if liberated and recoverable minerals, such as nahcolite, dawsonite, and trona, are inherent in the oil shale. Commercial in situ combustion of shale undergoing retorting causes partials of the retorting bed to acquire temperatures in excess of 1800.degree. F. to thereby fuse the minerals into an agglomerate. Agglomeration of the minerals interferes with efficient physical mineral separation.